I have had various numbers of piggies at one time. Currently I have 3 boys. when I introduced Rory, the third piggy, the whole hierarchy went out of the window and they had to sort out a new one. Once Rory hit his teens it was shaken up again and now they are adults they seem to have settled down.
Bobby has always been the boss of the 3, although as he gets older he is certainly a little calmer! can still be grumpy though, but that's got nothing to do with the other piggies, well, most of the time it doesn't!
Billy was originally in second place, especially when he could boss Rory around. However, he is now in third place and quite happy there. He will spend time with the others, all squashed together in their bedroom. he will spend time alone, up or downstairs, and he will spend some time with Rory, although even though Bobby is his brother they rarely spend time together. But I think he likes having his own time to chill, keep his thoughts in order and eat the carrots.
Rory was at the bottom when he first arrived, although he was a baby and bossed around by the others. Puberty changed that and he is now in second place. He will spend time Billy but is much closer to Bobby. The two of them are often found fluffed up together at the edge of the cage waiting for food.
you have to let the piggies sort out their own hierarchy. Its one of those things you cannot do for them. Some piggies will find they like to be alone at least some of the times and that is ok. Keep an eye on them to make sure that it isn't depression and give them lots of love, but also give the piggies time to work out who goes where and how the new friendships will work. It's ok for things to change.
as for depression, yes piggies can get it. They also pine when their best friend dies and can die pining. I know because it happened to one of my pigs. The pig concerned had been with one piggy for nearly 7 years but at the end his best friend was a little 2 1/2 year old piggy! I would never have put the 2 of them together but they found themselves!
trust that your piggies will sort themselves out, that it might take time and they might change. That's all ok!